Single carrier wireless telecommunications systems are known. In those known systems, radio coverage is provided to user equipment, for example, mobile telephones, by geographical area. A base station is located in each geographical area to provide the required radio coverage. User equipment in the area served by a base station receives information and data from the base station and transmits information and data to the base station.
Information and data transmitted by the base station to the user equipment occurs on channels of radio carriers known as downlink carriers. Information and data transmitted by user equipment to the base station occurs on uplink data channels of radio carriers known as uplink carriers.
In known single carrier wireless telecommunication systems, user equipment can move between geographical base station coverage areas. Service provided to user equipment is overseen by a radio network controller (RNC). The radio network controller communicates with user equipment and base stations and determines which base station each user equipment is primarily connected to. Furthermore, a radio network controller acts to control and communicate with a base station and user equipment when user equipment moves from the geographical area served by one base station to a geographical area served by another base station.
The area served by a given base station typically comprises several sectors which together define a coverage area for that base station. Typically a base station serves three sectors. Those sectors are typically served by separate antenna arrays provided on the base station. As well as controlling movement between base station coverage areas, the radio network controller also has visibility of, and communicates with, a base station and user equipment when user equipment moves between sectors within a coverage area served by a single base station.
Movement between sectors provided by a single base station and movement between coverage areas, that is to say movement between base stations are known as “mobility events” or “handover” and are overseen by the radio network controller. In a single carrier telecommunications system, those mobility events are notified to and controlled by the radio network controller via a single carrier.
It is proposed to allow base stations and user equipment to each transmit simultaneously on more than one carrier. Furthermore it is proposed to allow user equipment and base stations to receive simultaneously on more than one carrier frequency. Each carrier, both uplink and downlink, is typically independently power controlled and independently scheduled by a base station. Dual cell high speed uplink packet access (DC-HSUPA) will allow user equipment to transmit data on two adjacent frequency carriers. This allows for an increase in data throughput from the user equipment. Networks having more than two carriers may be referred to as “multi cell high speed uplink packet access” (MC-HSUPA) networks. The term “multi-carrier” network used herein is envisaged to cover both DC-HSUPA and (MC-HSUPA) networks.
In a typical multi-carrier wireless telecommunications network one of the carriers provided is known as an “anchor” or “primary carrier”. The anchor carrier has a particular functional purpose, and whilst a particular user equipment may be able to receive and transmit on one or more carrier simultaneously, the anchor uplink and downlink carriers are the carriers upon which a radio network controller monitors, controls and instructs the “mobility events” referred to above. A radio network controller relies upon uplink and downlink anchor carriers to maintain full visibility and control of a particular user equipment. That is to say, the anchor carrier provides essential control channels for downlink operation and mobility events are managed based upon measurements made by user equipment and reported on the anchor carrier uplink. It can be therefore understood that if an anchor carrier downlink radio link fails, user equipment may be forced to declare radio link failure even if a further carrier downlink radio link is available. If there is no anchor carrier a network is unaware of the condition of user equipment. In the event that radio link failure is declared, the user equipment may reconnect to the network via a different carrier to regain a HSUPA throughput. That reconnection process takes time.
Accordingly it is desired to improve the resilience of a multi-carrier wireless telecommunication network in the event of anchor carrier failure.